In general, this type of oil seal has at its outer peripheral portion a metallic reinforcement ring adapted to be fitted to a stationary part such as a housing and at its inner peripheral portion a rubber lip adapted to be made into sliding contact with a rotary shaft.
In the conventional oil seals of the type described, metallic reinforcement rings are made of cold rolled sheet steel and are designed and constructed so as to have a predetermined fitting allowance so that they are relatively easily fitted into the housings and are securely maintained in position by a fitting force (pulling load) which can sufficiently prevent the metallic reinforcement rings from easily being pulled out of the housings.
However, when an oil seal of the type described is fitted into a housing and is securely retained in position, even when the outer peripheral portion of a metallic reinforcement ring which is made of cold rolled sheet steel and is 0.5 mm in thickness and 100 mm in outer diameter has a maximum fit allowance (interference) of the order of 0.4 mm with which the metallic reinforcement ring can be fitted into the housing as indicated by the curve B in FIG. 2, there arises the problem, the residual fit allowance (that is, an allowance measured immediately after the metallic reinforcement ring is pulled out of the housing after it has been fitted therein) drops to about 0.08 mm due to the plastic deformations of the metallic reinforcement ring immediately resulting of the fitting of the metallic reinforcement ring into the housing.
When a housing is made of a material having a high coefficient of thermal expansion such as an aluminum alloy or a synthetic resin as compared with iron, the inner diameter of the housing is expanded greater than the outer diameter of the steel reinforcement ring as the temperature of the housing rises as indicated by the curve D in FIG. 3 so that the residual fit allowance is gradually decreased and becomes zero at about 105.degree. C. and consequently the steel reinforcement ring becomes loose relative to the housing. As a result, there arise the problems that the oil leaks through the space or clearance between the housing and the steel reinforcement ring and that the steel reinforcement ring is forced to be pulled out of the housing.
Therefore, the conventional oil seals of the type in which the outer periphery of a metallic reinforcement ring is made into contact with the bore surface of a housing can be used only in conjunction with the housings made of a steel or the like having a coefficient of thermal expansion substantially same as a metallic reinforcement ring at a high temperature range. Furthermore, as described above, the residual fit allowance is significantly decreased immediately after the fitting of mating parts so that in order to maintain a suitable mating force immediately after the fitting of mating parts, thick cold rolled sheet steel must be used. In this case, a designed fit allowance must be maintained with a high degree of accuracy. More specifically, as indicated by the curve F in FIG. 4, the permissible tolerance range of a fit allowance relative to the permissible to tolerance range of a suitable fitting or mating force becomes narrow. As a result, in addition to the convetional press bending step, an additional step such as grinding step is needed in order to maintain a desired degree of accuracy, resulting in the increase in fabrication cost. At present, therefore, it is extremely difficult to maintain a desired degree of dimensional accuracies while avoiding the increase in fabrication cost.
Since it is extremely difficult to maintain a desired degree of dimensional accuracy of the metallic reinforcement rings as described above, there has been devised and demonstrated an oil seal with a rubber periphery in which a rubber layer is baked around the outer periphery of the metallic reinforcement ring so that the dimensional errors of the metallic reinforcement ring are compensated for by the elasticity of the rubber layer and consequently even when the dimensional accuracies of the metallic reinforcement ring are relaxed, a predetermined fit allowance can be maintained. However, a coefficient of thermal expansion of rubber is extremely higher than that of a metal and when the outer peripheral rubber layer of the metallic reinforcement ring is heated at high temperatures when the metallic reinforcement ring is fitted into the housing, the rubber layer is significantly compressed between the housing and the metallic reinforcement ring because there is no space available between them for permitting the expansion of the rubber layer. As a result, the elastic deformations of the rubber layer result so that when the rubber layer is cooled, the residual fit allowance is decreased, resulting in the oil leakage.
In addition, in case of the molding of the rubber layer around the outer periphery of the metallic reinforcement ring, rubber is made to flow not only into an inner peripheral seal lip portion but also the outer periphery from the bottom portion of the metallic reinforcement ring concurrently so that the quantity of rubber used is increased, resulting in the increase in both the fabrication cost and the weight of the oil seal. Furthermore, a high molding pressure is needed to form the rubber layer, the number of the rubber layers molded in the same die is less and moreover there is the problem that the percent of the defective fraction is high.
Recently the performance of the automotive engines is increasingly improved so that the oil seals having a high degree of resistance to heat capable of withstanding a temperature range from about 150.degree. C. to 200.degree. C. As a result, it becomes necessary to use high-quality and considerably expensive rubber such as fluoro rubber. Meanwhile the automotive engines are made of light metal alloys such as an aluminum alloy having a high coefficient of thermal expansion in order to make the engines light in weight. As a consequence, the oil seals must be made light in weight and must follow the thermal expansion of the engine. Although the requirements for oil seals become severe as described above, the costs of oils seals must be as low as possible.